1. Field of the Invention
The present invention relates to a sheet feed device including a sheet storage member to store multiple sheets and a unit to move the sheet storage member up and down, and relates to an image forming apparatus including the sheet feed device.
2. Description of the Related Art
In recent years, a large-capacity sheet feed tray capable of storing more than a thousand sheets may be provided in a sheet feed device installed in an image forming apparatus. In such a sheet feed device, a sheet feed member including a roller pair or the like feeds the sheets one by one in order from the topmost sheet to the image forming apparatus.
In this type of the sheet feed device, the sheet feed tray is drawn from the sheet feed device to supply or replace sheets. In this case, the sheet feed tray moves down to the lowermost position due to the weights of stacked sheets. Therefore, it becomes possible to easily supply or replace sheets.
In the sheet feed device configured as described above, if the sheet feed tray is drawn while a large number of sheets are left in the sheet feed tray, a bottom plate of the tray moves downward due to the weights of the sheets before the tray is drawn. Therefore, in some cases, the sheet feed tray may move downward rapidly. To prevent such a situation, there is a known sheet feed device including a buffer mechanism (damper) to prevent the bottom plate of the tray from moving downward rapidly when the sheet feed tray is drawn (see, for example, Japanese Laid-open Patent Publication No. 2008-184300 and Japanese Laid-open Patent Publication No. 6-156754). In the conventional sheet feed device as described above, a predetermined load torque is applied to the bottom plate of the sheet feed tray when the bottom plate moves downward.
In the conventional sheet feed device including the damper as the buffer mechanism, a value of a load resisting force of the damper may be set on the assumption that a small number of sheets are stacked. In this case, if a large number of sheets are stacked, the load resisting force of the damper is so weak that the bottom plate of the tray may move downward rapidly, resulting in noise, such as an impact sound, or damage in the apparatus. In contrast, the value of the load resisting force of the damper may be set on the assumption that a large number of sheets are stacked. In this case, the load resisting force of the damper strongly acts when a small number of sheets are stacked. Therefore, the bottom plate may not move down to the predetermined lowermost position, and a user may need to manually move the bottom plate downward to supply or replace sheets.
Incidentally, while a rotary damper is used as the buffer mechanism in the technologies disclosed in Japanese Laid-open Patent Publication No. 2008-184300 and Japanese Laid-open Patent Publication No. 6-156754, there is also a known structure, in which a direct acting damper including a piston rod/cylinder is directly arranged in an upright manner in the sheet feed tray. Even in this case, the bottom plate may move downward by its own weight when a small number of sheets are stacked. In contrast, when a large number of sheets are stacked, it is possible to prevent the bottom plate from moving downward rapidly by applying a load resisting force to the bottom plate by the direct acting damper.
However, to directly arrange the direct acting damper in an upright manner in the sheet feed tray, it is necessary to increase a buffer stroke. Therefore, it becomes necessary to use a long-shaped damper, and a large space is needed inside the sheet feed tray.
In view of the above circumstances, there is a need to provide a sheet feed device capable of preventing a bottom plate of a tray for stacking multiple sheets from moving downward rapidly at low costs, and to provide an image forming apparatus including the sheet feed device.